1. No category

Toramatsu et al. Radiation Oncology 2013, 8:48
http://www.ro-journal.com/content/8/1/48
RESEARCH
Open Access
What is the appropriate size criterion for proton
radiotherapy for hepatocellular carcinoma? A
dosimetric comparison of spot-scanning proton
therapy versus intensity-modulated radiation
therapy
Chie Toramatsu1, Norio Katoh2*, Shinichi Shimizu2, Hideaki Nihongi1, Taeko Matsuura1, Seishin Takao1,
Naoki Miyamoto1, Ryusuke Suzuki1, Kenneth Sutherland1, Rumiko Kinoshita2, Rikiya Onimaru2, Masayori Ishikawa1,
Kikuo Umegaki1 and Hiroki Shirato2
Abstract
Background: We performed a dosimetric comparison of spot-scanning proton therapy (SSPT) and intensitymodulated radiation therapy (IMRT) for hepatocellular carcinoma (HCC) to investigate the impact of tumor size on
the risk of radiation induced liver disease (RILD).
Methods: A number of alternative plans were generated for 10 patients with HCC. The gross tumor volumes (GTV)
varied from 20.1 to 2194.5 cm3. Assuming all GTVs were spherical, the nominal diameter was calculated and ranged
from 3.4 to 16.1 cm. The prescription dose was 60 Gy for IMRT or 60 cobalt Gy-equivalents for SSPT with 95%
planning target volume (PTV) coverage. Using IMRT and SSPT techniques, extensive comparative planning was
conducted. All plans were evaluated by the risk of RILD estimated using the Lyman-normal-tissue complication
probability model.
Results: For IMRT the risk of RILD increased drastically between 6.3–7.8 cm nominal diameter of GTV. When the
nominal diameter of GTV was more than 6.3 cm, the average risk of RILD was 94.5% for IMRT and 6.2% for SSPT.
Conclusions: Regarding the risk of RILD, HCC can be more safely treated with SSPT, especially if its nominal
diameter is more than 6.3 cm.
Keywords: Spot-scanning proton therapy, Intensity-modulated radiation therapy, Hepatocellular carcinoma,
Radiation induced liver disease
Background
Unresectable primary and metastatic liver cancer is a
frequent cause of morbidity and mortality. Focal liver
radiotherapy (RT) can be used as a treatment option and
technological advancements have facilitated the safe use
of highly dose-conformal RT in liver cancers. However,
RT for large liver cancers is still challenging because of
the liver’s low tolerance dose for radiation-induced liver
* Correspondence: [email protected]
2
Department of Radiation Medicine, Hokkaido University Graduate School of
Medicine, Kita-15Nhisi-7, Kita-ku, Sapporo 060-8638, Japan
Full list of author information is available at the end of the article
disease (RILD) [1,2]. RILD is the most common liver
toxicity following RT [3,4]. Sparing of normal tissue
(normal liver) is severely required, and this limits the role
of RT for treatment of unresectable hepatic malignancies.
The widespread availability of intensity-modulated radiation therapy (IMRT) allows one to achieve significant
improvements in dose distributions for partial liver irradiation. IMRT inverse planning can generate complex
spatial dose distributions that closely conform to the target while sparing the organ at risk (OAR). However, the
downside to IMRT is the larger volume of normal tissue
© 2013 Toramatsu et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the
Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
Toramatsu et al. Radiation Oncology 2013, 8:48
http://www.ro-journal.com/content/8/1/48
Page 2 of 8
X-ray radiotherapy targeting the PVTT alone, not the
whole tumor, as a palliative treatment.
exposed to lower radiation doses. This can increase the
odds of developing RILD [5].
Proton beams are known to have superior normal tissue sparing compared with photons. Protons have a finite
range, which generally leads to an improved dose distribution, and this physical property can be used more effectively for the treatment of large volume tumors when
they are surrounded by critical organs [6]. Sugahara et al.
reported that proton RT was effective and safe for patients with hepatocellular carcinoma (HCC) greater than
10 cm in maximal dimensions [7].
Although proton RT has an advantage compared with
photon RT, to the best of our knowledge, there are no
available data on the appropriate size criterion for radiotherapy using proton beams for HCC. In this study, we
performed a dosimetric comparison of IMRT and spotscanning proton therapy (SSPT) for HCC to investigate
the impact of tumor size on the risk of RILD using the
normal-tissue complication probability (NTCP) model.
Treatment planning
In order to compare large volume irradiation, a GTV encompassing the whole tumor was re-contoured by a radiation oncologist using one of two treatment planning
systems: (TPS) XiO (CMS Inc., St Louis, MO), Pinnacle3
(Philips, Inc., Madison, WI) or Eclipse TPS Ver.10.0.0
(Varian Medical Systems, Palo Alto, CA). The clinical target volume (CTV) was defined as a 5 mm expansion
of the GTV minus areas of overlap with uninvolved
extra hepatic structures (e.g., lung, abdominal wall,
intestine) [8]. A 1 cm margin for set-up margin and
organ motion was uniformly added to the CTV to generate the planning target volume (PTV), assuming the use
of gated irradiation. Target and normal structures were
formatted and transferred to the Eclipse TPS Ver.10.0.0.
Here, for patients with extremely large targets (PTV
>1500 cm3), we prepared two different plans: one with
the GTV contoured PVTT alone (plan ID 1, 2 and 3 in
Table 1) and the other with the GTV contoured whole
tumor area (plan ID 11, 12 and 13 in Table 1). This is because we were concerned that patients with large tumors
should receive RT targeting the PVTT alone, even
though SSPT can spare the surrounding normal liver. A
total of 13 GTVs in 10 patients were used for IMRT
plans and SSPT plans (total of 26 plans were generated),
in this comparative study. We made a comparative
dosimetric study between simulated plans of photon
and proton beams on various irradiation volumes of
Methods and materials
Patients
We obtained approval from our institutional review
board at Hokkaido University Hospital for this retrospective dosimetric study. Between January and October
2011, 10 consecutive patients with HCC treated at our
institute, with GTV greater than 6 cm in diameter and
portal vein tumor thrombosis (PVTT), were included in
this study. The tumor size and that location data are
summarized in Table 1. CT data sets were acquired using
a slice thickness of 2 or 2.5 mm. All the patients received
Table 1 Targets and normal liver dimensions
PTV
(cm3)
GTV
(cm3)
Max. diameter
of GTV (cm)
Nominal diameter
of GTV (cm)
Normal
liver (cm3)
Overlapping of PTV and
normal liver (cm3)
The right main branch of the portal
vein
204.7
20.1
4.7
3.4
1660.6
142.0
2
The main trunk and the main
branches of the portal vein
348.0
59.2
7.2
4.8
2645.6
189.0
3
The main trunk and the main
branches of the portal vein
341.6
70.3
8.5
5.1
2658.7
302.3
4
Segment 3, 4
381.7
82.1
6.8
5.4
1783.5
207.8
Plan ID.
Main location of tumor
1
5
Segment 4, 8
405.9
130.9
7.3
6.3
814.6
300.3
6
Segment 1, 4, 6, 7, 8
844.8
245.8
15.1
7.8
1312.6
310.2
7
Segment 7, 8
764.2
284.2
12.4
8.2
1216.5
367.3
8
Segment 3, 4, 5, 7, 8
785.6
299.3
10.3
8.3
1339.9
411.2
9
Segment 3, 4, 5, 7, 8
848.9
344.5
13.8
8.7
1676.9
399.7
10
Segment 6, 7, 8
1488.3
720.6
15.2
11.1
965.4
489.4
11
Segment 1, 5, 6, 7, 8
1822.4
916.0
15.4
12.1
1088.7
563.2
12
Segment 1, 4, 5, 6, 7, 8
2222.6 1638.9
18.0
14.6
1087.5
621.7
13
Segment 1, 4, 5, 6, 7, 8
3094.2 2194.5
22.0
16.1
629.2
630.1
Mean
-
-
8.6
1452.3
379.6
1042.5
539.0
Toramatsu et al. Radiation Oncology 2013, 8:48
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HCC utilizing IMRT and SSPT plan techniques. These
simulated plans were generated in Eclipse TPS, assuming
photon treatment with an MHCL-15SP v80m (Mitsubishi
Electronics Co., Ltd., Tokyo) LINAC and proton treatment with a Probeat III (Hitachi Co Ltd, Japan) proton accelerator [9], respectively.
For the IMRT plans, five to nine evenly spaced intensity
modulated fields were generated with a 6 MV photon
beam. For Proton plans, the SSPT technique [10], which
can be generated using the inverse planning approach like
IMRT, was applied. For SSPT plans, simple arrangements
of two or three proton fields were selected. The dose distributions of proton plans generated with only two of
three fields were dependent on field incidence, achieving
good results for the normal liver spared by beam entrance.
First, one beam angle was selected so that beam paths can
take the shortest way to cover the target. Then one or two
more beams were added and each beam’s angle was adjusted so that dose distribution was homogenous. The
SSPT plans were simulated as individually weighted proton Bragg peaks distributed throughout the PTV. Energies
were selected from those actually deliverable. The energies
required to cover the target homogeneously were 70 to
180 MeV. For SSPT plans, a relative biologic effectiveness
factor for protons of 1.1 was employed.
The dose prescription for this study was 60 Gy for IMRT
plans or 60 cobalt Gy equivalents (CGE) for SSPT plans
covering 95% of the PTV, delivered in 15 fractions [11,12].
The dose volume constraints for the liver used in the planning process were taken on the basis of normal tissue tolerances as estimated by Emami et al. [13]. The maximum
doses delivered to one third and two thirds of the normal
liver were planned in order not to exceed the tolerance
dose (TD) 5/5 (the probability of 5% complications within
5 years from irradiation). The constraint for the normal
liver was set at its volume receiving 33 Gy or CGE or more
(V33) less than 67% and V42 less than 33%. We also considered the dose-volume limits for therapeutic partial liver
RT which is recommended by Pan CC et al. [14]. Mean
normal liver doses were planned in order not to exceed
the tolerance dose 28 Gy (CGE) in 2 Gy or CGE fractions
for primary liver cancer. Dose limits of 33, 42 and 28 Gy
(CGE) in 15 fractions were specified. These are equivalent
to 35, 50 and 28.6 Gy (CGE) using 2 Gy (CGE) per fraction assuming an α/β ratio of 2.5, respectively. Both IMRT
and SSPT plans were optimized with the requirement that
at least 95% of the PTV received the prescribed dose.
Evaluation
Once an acceptable treatment plan was obtained, dosevolume histogram (DVH) analysis was performed. First,
DVH comparisons of the IMRT and SSPT plans were
made. Although many representations of the data are
valid, we used the Lyman-Kutcher-Burman (LKB) NTCP
Page 3 of 8
model [15-17] to calculate the risk of RILD. The LKBNTCP model is used to estimate the volume dependence
of normal tissue toxicity that permits comparisons between plans based on DVHs. The Lyman-NTCP model
describes the probability of a complication after uniform
radiation of a fractional volume of normal tissue (v) to a
dose (D), as
2
Z t
1
x
NTCP ¼ φðt Þ ¼ pffiffiffiffiffiffi
dx
ð1Þ
exp 2
2π 1
where
TD50 ðvÞ
t ¼ D
m:TD50 ðvÞ
ð2Þ
TD50(v) represents the tolerable dose associated with
a 50% chance of complications for uniform partial liver
irradiation, where TD50(v) is related to the whole liver
(v = 1) tolerance through the power law relationship:
TD50 ðvÞ ¼ TD50 ð1Þ:vn
ð3Þ
TD50(1) represents the tolerance of the whole organ to
irradiation, m characterizes the steepness of the doseresponse at TD50(1), and n represents the volume effect,
which relates the tolerance doses of uniform whole
organ irradiation to uniform partial organ irradiation.
Dawson and colleagues derived these three parameters
as n = 0.97, m = 0.12, and TD50(1) = 39.8 Gy (CGE)
(hereafter referred to as the Michigan parameters) from
the LKB-NTCP model fitted to the complication data
for 203 patients with liver cancer treatment, 11 fr/wk,
1.5–1.65 Gy (CGE) /fraction [3]. The median dose of radiation delivered was 52.5 Gy (CGE) (range 24–90). In
this study, the risk of RILD was estimated using the
LKB-NTCP model with the Michigan parameters.
DVH normalization
To correct for the difference in protocol between this
study and the Michigan study [3], normal liver DVHs
were normalized before computation of the dose distributions from which the DVHs were computed. The
physical dose values of each plan were converted to normalized iso-biologic effective doses using the linear
quadratic model (LQ-model). That is, the normal liver
cumulative DVH dose bins were converted to a fractionsize equivalent dose (FED) [18] described as
d
nd
1
þ
α=
f
β
FEDα =sβ ≡
ð4Þ
fs
1 þ α=
β
where fs denotes a reference fraction size. For example, the dose per fraction delivered using the schedule
Toramatsu et al. Radiation Oncology 2013, 8:48
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Page 4 of 8
from which the modeled NTCP data was derived, d, is
the physical dose per fraction, n is the total number of
fractions, and α/β is the ratio of the linear LQ-model parameters for the organ and end point at risk. The mean
FED is then described as
f
Mean FEDα =s ¼
β
XNb h
n¼1
i
f
FEDα =s β vi
ð5Þ
i
so that it is adjusted to one of smaller volume ΔVeff
and extension Dmax using
1 =n
ΔVeff i ¼ Δvi Di Dmax
ð6Þ
where n is a size parameter. This procedure is applied
to each bin of the histogram. The normalized normal
liver DVH with a reduction of the DVH to the effective liver volume is then irradiated as
where Nb denotes the total number of dose bins in the
fs
differential DVH, [FED α/β
]i is the FED in the i-th dose
bin, and v i isP
the partial volume associated with the ith dose bin ( Nb
n=1 v i =1). In this study, we calculated
the FED, continuing to work with the reference point
dose per fraction fs and α/β ratio for which Dawson’s
NTCP model fit derived, following the treatment
schedule employed by Dawson et al. [3], and using an
α/β = 2.5.
The LKB-NTCP was then calculated using the Michigan
parameters and the normalized normal liver DVH with a
reduction of the DVH to the Veff.
Effective volumes
Statistical analysis
To convert the non-uniform complex dose distributions
into “equivalent” uniform dose distributions, the KutcherBurman effective volume (Veff )-DVH reduction scheme
[16] was used. The Veff method transforms the histogram
into a uniform histogram of height Veff and dose, Dmax,
equal to the maximum dose in the histogram. This transformed histogram is assumed to indicate the same
probability of complications as the original histogram.
Each step in the histogram of height Δvi and extension Di is assumed to satisfy a power law relationship
Mean FED, Veff and the risk of RILD for normal liver were
calculated and compared between IMRT plan and SSPT
plan. We investigated the size of GTV that could be delivered at a prescribed dose for targets with successful
achievement of dose constraints to the normal liver. Assuming all GTVs were spherical, the nominal diameter
was calculated and used in this study. The median volume
of GTV was 539.0 cm3 (range, 20.1–2194.5) and the median nominal diameter was 8.6 cm (range, 3.4–16.1) as
shown in Table 1.
1 =n
Veff ¼ Δv max þ Δv1 D1 Dmax
1 =n
þ Δv2 D2 Dmax
þ⋯
ð7Þ
Table 2 Values of Veff and mean FED
ID.
Nominal
diameter
of GTV
(cm)
V33
V*eff
V42
Mean FED*
DMean
IMRT (%) SSPT (%) IMRT (%) SSPT (%) IMRT SSPT p-value IMRT (Gy) SSPT (Gy) IMRT (Gy) SSPT (Gy) p-value
1
3.4
21.4
19.3
18.1
15.9
0.56
0.29
24.4
16.3
35.9
20.2
2
4.8
26.4
19.1
20.8
14.2
0.49
0.30
21.4
16.4
33.1
20.1
3
5.1
18.7
15.9
15.5
14.5
0.46
0.25
19.8
13.6
30.3
16.6
4
5.4
25.4
17.8
17.9
17.9
0.40
0.24
24.2
17.7
32.8
15.2
5
6.3
31.2
21.2
23.2
20.2
0.47
0.33
37.9
22.4
37.9
19.9
6
7.8
61.8
30.0
40.9
28.3
0.70
0.47
36.9
27.4
52.5
33.1
7
8.2
67.5
27.3
59.7
27.3
0.72
0.49
36.1
28.1
54.5
32.5
8
8.3
76.9
41.2
42.7
27.7
0.62
0.32
32.7
17.4
56.7
24.2
38.3
28.6
58.1
33.3
38.8
24.4
67.9
27.3
0.021
9
8.7
70.0
45.6
60.4
27.9
0.75
0.53
10
11.1
61.3
39.8
60.2
28.2
0.76
0.44
11
12.1
89.5
50.9
67.3
30.9
0.83
0.57
41.9
31.7
75.4
32.4
12
14.6
70.9
48.2
55.3
29.7
0.73
0.45
37.1
25.6
64.2
30.4
13
16.1
64.7
48.5
62.4
37.9
0.86
0.55
42.4
30.3
78.4
31.5
ave.
8.6
52.7
32.7
41.9
24.7
0.64
0.40
33.2
23.4
52.1
25.9
< 0.001
*The values of Veff and mean FED were normalized to 1.5 Gy per fraction, assuming an α/β of 2.5.
0.012
< 0.001
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The mean FED for normal liver was checked against the
Michigan study [3] in which Dawson et al. [3] reported
that no cases of RILD were observed when the mean liver
dose was under 31 Gy, and the mean liver doses associated with a 5% risk of RILD for patients with metastatic
and primary liver cancer are 37 Gy and 32 Gy, respectively
(in 1.5 Gy per fraction, assuming an α/β = 2.5 for the
liver). The values of mean FED, Veff and the risk of RILD
for normal liver were aligned as a function of the nominal
diameter of GTV. For the values of mean FED and Veff,
the Wilcoxon signed-rank test was performed to compare
differences between IMRT and SSPT plans, with a p value
of < 0.05 being considered significant.
Results
DVH comparison among plans
For all 10 patients, a total of 26 plans for each technique
of IMRT and SSPT were evaluated through careful review
of the PTV isodose distributions on all slices by a radiation oncologist. Each plan provided good coverage of
the target, covering 95% of the PTV. For the patients with
relatively small targets having nominal diameters less than
or equal to 6.3 cm (volume of GTV ≤ 130.9 cm3), the target dose optimization according to dose constraints to
one third and two thirds of the volume of the normal liver
was successfully achieved for IMRT plans (Table 2). However, when the nominal diameter was greater than or
equal to 7.8 cm, none of the IMRT plans were able to deliver a prescribed dose of 60 Gy covering 95% of the PTV
for larger targets without sacrificing normal liver. SSPT
plans achieved dose constraint for the normal liver successfully in all plans except for the plan with a 16.1 cm
nominal diameter GTV (Table 2). SSPT plans obtained
clearly superior results to IMRT plans for every case. Examples of the DVH and dose distributions obtained with
IMRT and SSPT plans for the cases of nominal diameter
of GTV = 5.1 cm (a), 7.8 cm (b) and 16.1 cm (c) are shown
in Figure 1 and Figure 2.
Estimation of RILD
The mean FED of normal liver, summarized in Table 2, increased with the size of the GTV. The averages of the
mean FED of normal liver for IMRT plans and SSPT plans
were 52.1 Gy (range, 30.3–78.4) and 25.9 CGE (range,
15.2–33.3), respectively. SSPT plans maintained a lower
mean FED of normal liver that of IMRT plans (p =0.01)
The mean FEDs for SSPT plans reached a plateau at
around 30 CGE, while that for IMRT plans increased with
the size of the tumor. The values of Veff for each plan are
also summarized in Table 2. The mean values of Veff for
normal liver in IMRT plans and SSPT plans were 0.64
(range, 0.40–0.86) and 0.42 (range, 0.29–0.60), respectively. SSPT achieved a lower Veff for normal liver than did
IMRT ( p < 0.001 ). The Veff for normal liver of the SSPT
Figure 1 Cumulative dose-volume histograms (cDVH) of normal
liver for IMRT plans (solid line) and SSPT plans (dashed line).
(a) cDVH for patients with nominal diameter of GTV (a) 5.1 cm,
(b) 7.8 cm and (c) 16.1 cm. Triangles in each figure were dose constrains
for one-third and for two-thirds of the volume of the normal liver.
reached a plateau around 0.5 even if the size of the tumor
increased while that for IMRT plans increased with tumor
size. For IMRT plans, both the mean FED and Veff for normal liver drastically increased from between 6.3–7.8 cm
Toramatsu et al. Radiation Oncology 2013, 8:48
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Figure 2 Dose distributions obtained with IMRT plans and SSPT plans for patients with nominal diameter GTV of (a) 5.1 cm, (b) 7.8 cm
and (c) 16.1 cm.
nominal diameter of GTV, and the difference between
IMRT and SSPT plans was significant when the nominal
diameter of GTV was more than 6.3 cm (Table 2).
Figure 3 shows the relationship between risk of RILD
probability and tumor size. As expected, the risk of RILD
probability for HCC for all plans increased in relation with
the value of Veff and mean FED for normal liver; that for
IMRT varied from 0.03 to 1.00, while that for SSPT varied
from 0.0003 to 0.087. IMRT plans were able to keep the
risk of RILD low in cases of GTV nominal diameter below
6.3 cm, but then the risk increased drastically with tumor
size. For tumors of GTV nominal diameter less than or
equal to 6.3 cm, the averages of RILD probability for
IMRT and SSPT plans were 0.016 and 0.009, respectively.
For GTV nominal diameter greater than 6.3 cm, the average RILD probabilities for IMRT and SSPT plans were
0.942 (range, 0.822–1.00) and 0.045% (range, 0.001–
0.087), respectively. The RILD probabilities were lower
than IMRT when using SSPT for all cases, and they were
significantly lower for nominal diameters of GTV greater
than 6.3 cm.
Discussion
In this study, the SSPT technique was applied. The scanning target volume, an optimization volume for SSPT
planning, should be defined for each liver cancer patient
using the distal margin including the range uncertainties
defined by dm = 0.035 × R + U, where R is the most distal range in cm for the CTV, and U is the beam range
uncertainty [19-21]. The beam range uncertainty for accelerator energy and for pre-absorber devices [22]
should be considered here, and expansions in any direction should be defined based on our own experience of
setup uncertainties, which would be similar to the margins used for IMRT [23]. Because our proton therapy
system is under construction these investigations of
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Figure 3 Relationship of the risk of RILD to the nominal diameter of GTV. Prescribed doses of 60 Gy for IMRT (black diamond) and 60GyE
for SSPT (open circle), delivered in 15 fractions, were normalized for all plans to 1.5 Gy b.i.d. assuming an α/β of 2.5, based on the LKB-NTCP
model (n = 0.97, m = 0.12, TD50(1) = 39.8 Gy ). The difference of the risk of RILD between IMRT and SSPT plans clearly increased from 6–7 cm
nominal diameter of GTV.
range uncertainty and margin for this area will be
presented in a future publication. In this study, a 1 cm
margin for set-up margin and organ motion was uniformly added to CTV to generate PTV. This margin recipe was based on a previous article [24].
Even using SSPT, the risk of RILD was sometimes
above 5% in this study. With the application of a realtime tumor tracking (RTRT) system [25] this margin can
be decreased. The combined SSPT and RTRT system is
expected to permit safe treatment of large moving liver
cancers. Investigation of the proper margin to generate
PTV from CTV in cases of SSPT with the application of
RTRT also will be a topic of future study.
In this study, estimations of RILD were calculated
using the LKB-NTCP model with the Michigan parameters. However, inherent biologic uncertainties are
present in all NTCP models. As the prescription for
proton therapy is very different from that used with the
liver for which the NTCP model was developed, the protocol used in this study is different from the Michigan parameters [3]. This study could not confirm the accuracy of
the risk of RILD, and we should note the possibility that
patients may have an impaired liver function because of
an underlying cirrhosis in the course of HCC and therefore even sub-RILD doses may lead to a severe deterioration of the liver function [26,27]. Future work is required
to better understand the partial volume tolerance of the
liver to SSPT. However, the risk of RILD for the SSPT plan
is confirmed to be low enough, and the estimations in this
study are still useful for safely guiding irradiation volume
allocation for clinical treatment cases. This study should
help to expand the understanding of dose response once
mature outcome data are available.
The risk of RILD may not only depend on tumor size
but also on the location, due to the integral dose to normal liver tissue in the entrance channels for proton
fields. In the clinical case, it is also required to evaluate
the safety of beam arrangement, prescription and fractionation schemes considering the tumor location. In
this study, we used unbiased data on tumor localization
as summarized in Table 1. Beam angles were selected
simply in terms of sparing healthy liver tissue. In this
dosimetric study, we investigated the impact of tumor
size itself on the risk of RILD. Our results show that if
the nominal diameter of GTV is more than 6.3 cm, average of the risk of RILD was 94.5% while that of SSPT
was 6.2%. Although the advantage of protons in sparing
the normal liver has been reported in several papers
[6,7], as far as we could survey, this is the first report to
investigate the impact of tumor size on the risk of RILD,
offering a clear reason why the advantage of proton therapy becomes effective from a nominal diameter GTV of
around 6 cm.
Conclusions
A comparative dosimetric study was undertaken between SSPT and IMRT. All plans were evaluated with
DVH-analysis and the risk of RILD was estimated. Regarding the risk of RILD, HCC can be more safely
treated with SSPT, especially if its nominal diameter is
more than 6.3 cm.
Toramatsu et al. Radiation Oncology 2013, 8:48
http://www.ro-journal.com/content/8/1/48
Abbreviations
SSPT: Spot-scanning proton therapy; IMRT: Intensity-modulated radiation
therapy; HCC: Hepatocellular carcinoma; RILD: Radiation induced liver
disease; GTV: Gross tumor volumes; CTV: Clinical target volume; PTV: Planning
target volume; NTCP: Normal-tissue complication probability; PVTT: Portal
vein tumor thrombosis; TPS: Treatment planning systems; CGE: Cobalt Gy
equivalents; TD: Tolerance dose; DVH: Dose-volume histogram; LKBNTCP: Lyman-Kutcher-Burman the normal-tissue complication probability.
Competing interests
There are no actual or potential conflicts of interest to disclose for any of the
authors.
Authors’ contributions
CT reviewed and analyzed the data, performed statistical analyses, created
the figures, and drafted the manuscript. NK reviewed and analyzed the data,
performed statistical analyses, and assisted in drafting the manuscript. SS
conceived and designed of the study. HN performed the statistical design
and analysis. TM helped data collection. ST assisted in drafting the
manuscript. NM performed dosimetric analysis for the manuscript. RS helped
data collection. KS assisted in drafting the manuscript. RK participated the
design of the study. RO assisted in drafting the manuscript. MI assisted in
drafting the manuscript. KU assisted in drafting the manuscript. HS drafting
of the manuscript with final approval of manuscript. All authors read and
approved the final manuscript.
Acknowledgements
This research is supported by the Japan Society for the Promotion of Science
(JSPS) through the “Funding Program for World-Leading Innovative R&D in
Science and Technology (FIRST Program),” initiated by the Council for
Science and Technology Policy (CSTP).
Author details
1
Department of Medical Physics, Hokkaido University Graduate School of
Medicine, Sapporo, Japan. 2Department of Radiation Medicine, Hokkaido
University Graduate School of Medicine, Kita-15Nhisi-7, Kita-ku, Sapporo
060-8638, Japan.
Received: 18 December 2012 Accepted: 24 February 2013
Published: 5 March 2013
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doi:10.1186/1748-717X-8-48
Cite this article as: Toramatsu et al.: What is the appropriate size
criterion for proton radiotherapy for hepatocellular carcinoma? A
dosimetric comparison of spot-scanning proton therapy versus
intensity-modulated radiation therapy. Radiation Oncology 2013 8:48.
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